1. Field of the Invention
The present invention relates to a method of producing a multi-layered hose which is suitable for producing fuel hoses, air intake hoses for automobiles and the like.
2. Description of the Related Art
Conventionally, rubber materials such as NBR-polyvinylchloride rubber which show superior oil resistance and oil penetration resistance are used for filler hoses (fuel hoses) for automobiles.
However, the above conventional filler hoses comprising only a single rubber layer cannot cope with recent severe regulations of oil penetration resistance.
As an attempt to meet the recent demand, as shown in FIG. 8(A), it has been suggested to produce a hose by forming a thin fluorine-contained rubber inner layer 102 on the inner surface of a rubber outer layer 100. Although this type of hose shows excellent performance, the production cost is very high. So, it is not suitable for practical use.
On the other hand, in the case of hoses for transmitting refrigerants, as shown in FIG. 8(B)(a), it is known to produce a hose by laminating a thin resin inner layer 104 on the inner surface of a rubber outer layer 100. By employing this hose structure and laminating the resin inner layer 104 having excellent gasoline resistance and excellent gasoline penetration resistance on the inner surface of the rubber outer layer 100, the gasoline resistance and gasoline penetration resistance of the hose can be increased at a low cost, while maintaining flexibility which is required for filler hoses.
In this case, a hard resin material is usually used as the resin inner layer 104, and the resin inner layer 104 is formed at a certain thickness which is required to secure gasoline resistance and gasoline penetration resistance. This is because hard resin materials show more excellent gasoline resistance than that of soft resin materials in general.
Conventionally, hoses having this multi-layered structure have been produced, for example, by the following method. As shown in FIG. 8(B)(b), a mandrel 106 is employed and a resin tube constituting the resin inner layer 104 is extrusion molded, and then a rubber tube constituting the rubber outer layer 100 is laminated on the outer surface of the resin tube by extrusion molding. Next, the obtained rubber tube is vulcanized in the case of producing a straight hose. Otherwise, the obtained rubber tube is semivulcanized, then bent in a certain direction, and after that finishes vulcanizing in the case of producing a bent hose.
However, it is difficult to employ this method in producing hoses having large inner diameters, because the mandrel 106 must be drawn out from the hoses after vulcanization but is hard to be drawn out. Further, it is difficult to employ this method in producing hoses having complicated shapes such as hoses having a bellows portion or complicatedly bent portions.
In the meanwhile, the resin constituting the resin inner layer 104 is preferably a hard resin material in view of gasoline resistance, as described in the above. Accordingly, in view of hose flexibility, the thickness of the resin inner layer 104 must be small. Here, it may be considered to form the resin inner layer 104 by injection molding. In the case of forming the resin inner layer 104 by injection molding, however, it is difficult to make the thickness of the resin inner layer 0.3 mm or less. Further, as disclosed in Japanese Unexamined Patent Publication (KOKAI) NO.275238/1991, it may be considered to form the resin inner layer 104 by a blow molding method, that is, by disposing a resin parison (form) in the inner space of the outer layer 100 and blowing an air into the parison so that the parison is swelled in a centrifugal direction and closely adheres to the inner surface of the outer layer 100. However, the accuracy of the thickness of the resin inner layer 104 cannot be secured sufficiently. Further, when the thickness of the resin parison is too small, the cooling speed is increased and it is difficult to maintain the resin in a softened state. So, there has been a limit in decreasing the thickness of the resin inner layer 104.